In the construction of most commercial buildings, a finished ceiling, commonly referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items are supported by the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for heating and cooling systems as well as a convenient location for the installation of communication, computer, and alarm system cables and/or cordage. Often, these plenums are continuous throughout the length and width of each floor.
When a fire occurs in the area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose the area. However, if the fire reaches the plenum, and if combustible material occupies the plenum, the fire can spread quickly throughout an entire story of the building and smoke can be conveyed through the plenum to adjacent areas. In addition, the fire could travel along the length of the cables installed in the plenum. Because of the possibility of flame spread and smoke evolution, particularly when aided by flammable cable or cordage insulation, the National Electric Code (NEC) requires that flammable cables used in plenums be enclosed in metal conduits. However, since rigid metal conduits are difficult to route in plenums congested with other items, the NEC also permits certain exceptions to the metal conduit requirement. For instance, low flame spread, low smoke producing cables are approved for plenum use without metal conduit provided that the cables are approved by an authority such as Underwriter's Laboratories.
Underwriter's Laboratories (UL) has developed a standard for flame propagation and smoke density values for electrical and optical fiber cables used in plenums called UL 910. UL 910 requires that, when tested in accordance with the specific procedures described in the written standard, the optical fiber cable produce (i) a maximum flame propagation distance of no more than five feet; (ii) a peak optical density of smoke no greater than 0. 50; and (iii) an average optical density of smoke of 0.15 or less.
Previously, compliance with UL 910 for optical fiber cable has been obtainable through use of substantially flame resistant cable jackets. Such jackets generally are considered necessary for compliance with UL 910 in that the optical fibers themselves normally are constructed of highly flammable coating materials. Despite the flame resistance provided by such jackets, the high flammability of conventional optical fibers makes complying with UL 910 difficult. Accordingly, those in the art have been searching for an optical fiber which satisfies all of the performance needs of modem communications systems and which is also flame resistant.
Conventional optical fibers typically include a glassy core, cladding, and one or more layers of a coating composed of an acrylate material. Surrounding the coating is at least one further layer of material, commonly referred to as a buffer or buffer layer, which protects the fiber from damage and which provides the appropriate amount of stiffness to the fiber. This buffer layer usually is mechanically stripped away from the fiber when the fiber is connected to an optical fiber connector. Normally, the buffer layer is composed of a thermoplastic polymeric material which is extruded directly over the coated optical fiber. Common materials used to form buffer layers include polyvinyl chloride (PVC), nylon, and polyesters. Of these materials, nylon and polyesters are used most frequently, especially in the manufacture of optical fiber cordage, due to their relatively high modulus. As is known in the art, high modulus is desirable because it facilitates connectorization of optical fibers. However, as is known in the art, nylon and polyesters are also highly flammable.
Although there are several known flame resistant materials currently available, there has been difficulty in the industry in finding a buffer material which provides both a high modulus and the requisite flame resistance. This difficulty is compounded by the fact that other optical fiber design requirements must similarly be satisfied by the selected buffer material. For instance, in addition to high modulus and flame resistance, the finished optical fiber must also satisfy transmission, manufacturing, chemical resistance, and strippability requirements. Satisfaction of each of these requirements greatly complicates the design and development process and has, until now, impeded production of high modulus, flame resistant buffered optical fibers.
From the foregoing, it can be appreciated that it would be desirable to have a high modulus optical fiber which is higher flame resistant and also satisfies transmission, manufacturing, chemical resistance, and strippability requirements so as to be suitable for use in the construction of plenum rated cables and cordage.